This invention relates to the treatment of a gas stream comprising hydrogen sulphide.
Sour gas streams containing hydrogen sulphide are, for example, formed as by-products of gas and oil refining operation. Examples of sour gas streams containing hydrogen sulphide are products of natural gas wells, tail gas streams for such industrial sources as the hydrodesulphurisation or hydrotreating units of an oil refinery or a plant for synthesis gas manufacture.
The sour gas stream typically contains less than 40% by volume of hydrogen sulphide, sometimes less than 10% by volume. Other gaseous components of sour gas streams typically include carbon dioxide, ammonia, and hydrocarbons. Conventionally, such gas streams are first concentrated and then treated by the Claus process. The concentration step typically comprises absorbing hydrogen sulphide in an aqueous solution of a suitable amine, and then desorbing the hydrogen sulphide from the aqueous solution. Typically, the resulting gas stream comprises at least 40% by volume of hydrogen sulphide, and frequently more than 70%. The resulting gas stream also contains carbon dioxide, the relative proportions of hydrogen sulphide and carbon dioxide depending on the selectivity of the chosen amine absorbent for hydrogen sulphide. Such gas streams are often referred to as xe2x80x9cacid gas streamsxe2x80x9d.
Conventionally, such acid gas streams are treated by the Claus process. An acid gas stream may be mixed upstream of treatment by the Claus process with a so-called sour water stripper gas stream, typically comprising hydrogen sulphide, water vapour, and ammonia. The Claus process typically includes an initial thermal stage in which part of the hydrogen sulphide content of the gas stream is subjected to combustion to form sulphur dioxide and water vapour. The sulphur dioxide reacts in the combustion furnace with residual hydrogen sulphide to form sulphur vapour and water vapour. The reaction between sulphur dioxide and hydrogen sulphide does not proceed to completion in the furnace. Typically two or three further stages of reaction between hydrogen sulphide and sulphur dioxide are required to achieve, say 98%, conversion to sulphur of the incoming hydrogen sulphide. The reaction in these further stages is catalysed, with sulphur vapour being removed from the gas steam upstream of each catalytic stage. Claus plants are therefore large installations employing large beds of catalysts. Modern environmental standards typically necessitate the achievement of higher conversion efficiencies than 98%. In order to meet these standards, a large xe2x80x9ctail gas clean up unitxe2x80x9d is typically added to the Claus plant.
Some reductions in the size of a Claus plant can be achieved if the gas that is used to support the combustion of part of the hydrogen sulphide is oxygen-enriched air rather than atmospheric air (unenriched in oxygen).
EP-A-565 316 relates to a process which is operable to reduce or eliminate the requirements for catalyst of the reaction between hydrogen sulphide and sulphur dioxide. The concept underlying most examples of the process according to EP-A-565 316 is that by recycling hydrogen sulphide to the furnace, a high effective conversion of hydrogen sulphide to sulphur can be achieved therein, thereby limiting the amount of catalytic reaction of hydrogen sulphide and sulphur dioxide downstream of the furnace. In order to form the hydrogen sulphide recycle stream, the gas stream from the furnace, downstream of a condenser for extracting sulphur vapour, is subjected to catalytic hydrogenation so as to reduce back to hydrogen sulphide all the sulphur dioxide present. Most of the water vapour is condensed out or otherwise removed from the reduced gas stream and the resulting water vapour depleted reduced gas stream is divided into two parts, one part being returned to the furnace, and the other part being subjected to further treatment, typically in an associated Claus plant of conventional kind. In order to maintain adequate temperatures in the furnace, the source of oxygen molecules which are used to support combustion therein is a source of oxygen-enriched air containing at least 80 mole % of oxygen and more preferably a source of commercially pure oxygen.
The problem remains however of being able to extract substantially all the (chemically combined) sulphur in the feed gas without requiring treatment of purge gas in such an auxiliary Claus plant and without necessitating a recycle rate to the furnace which is several times the rate at which feed gas enters the furnace.
The invention provides a method and apparatus aimed at addressing this problem.
According to the present invention there is provided a method of treating a sour gas containing hydrogen sulphide, comprising the steps of:
a) selectively absorbing hydrogen sulphide from the sour gas and from a recycle gas in a selective absorbent of hydrogen sulphide;
b) generating a feed gas stream containing hydrogen sulphide by stripping absorbed gas from the selective absorbent;
c) burning in a furnace part of the hydrogen sulphide content of the feed gas stream so as to form sulphur dioxide and water vapour, supplying oxygen-enriched air or oxygen to the furnace to support combustion of the said part of the feed gas, and reacting in the furnace resulting sulphur dioxide with hydrogen sulphide so as to form as effluent gas stream containing sulphur vapour, water vapour, hydrogen sulphide, and sulphur dioxide;
d) extracting the sulphur vapour from the effluent gas stream so as to form a sulphur-depleted gas stream;
e) reducing to hydrogen sulphide essentially the entire content of sulphur dioxide and any sulphur vapour in the sulphur-depleted gas stream so as to form a reduced gas stream;
f) removing most of the water vapour from the reduced gas stream so as to form a water vapour depleted gas stream; and
g) returning at least part of the water vapour depleted gas stream to said step a) as the recycle gas.
The invention also provides apparatus for the treatment of a sour gas containing hydrogen sulphide, comprising:
a) an absorber vessel operable to receive a selective absorbent of hydrogen sulphide and to absorb therein hydrogen sulphide from the sour gas and from a recycle gas;
b) a desorber vessel operable to receive from the absorber vessel the selective absorbent charged with gas, and to form by stripping gas from the selective absorbent a feed gas stream containing hydrogen sulphide;
c) a furnace arranged to burn in the presence of oxygen or oxygen-enriched air part of the hydrogen sulphide content of the feed gas so as to form sulphur dioxide and water vapour, and to allow reaction to take place between hydrogen sulphide and sulphur dioxide to form sulphur vapour and water vapour, the furnace having an outlet for an effluent gas stream containing sulphur vapour, water vapour, hydrogen sulphide and sulphur dioxide;
d) means for extracting sulphur vapour from the effluent gas stream and thereby forming a sulphur-depleted gas stream;
e) a reactor for reducing to hydrogen sulphide essentially the entire content of sulphur dioxide and any sulphur vapour in the sulphur vapour depleted gas stream entering the reactor, and thereby forming a reduced gas stream;
f) means for extracting from the reduced gas stream most of its water vapour content and thereby forming a water vapour depleted gas stream; and
g) a recycle gas passage leading from the water vapour extraction means to the absorber vessel.
By employing the same absorber to concentrate in hydrogen sulphide both the sour gas flow and a recycle gas flow the capital cost of the plant is kept down. A conventional Claus plant is typically located upstream of an initial gas treatment unit for concentrating the sour gas in hydrogen sulphide, and typically includes and a furnace with associated waste heat boiler and sulphur condenser for removing about two thirds of the incoming sulphur content of the hydrogen sulphide in the sour gas, a plurality of catalytic Claus stages for removing most of the residual sulphur content, and a tail gas clean up unit for extracting from the effluent gas that exits the most downstream of the catalytic Claus stages. In comparison, the method and apparatus omits the tail gas clean up unit and the catalytic Claus stages, employing only the reduction stage (which may as will be described below include a bed of Claus catalyst) and a water removal stage in their stead.
The absorbent of hydrogen sulphide is preferably an aqueous solution of an amine adapted for the selective separation of hydrogen sulphide from carbon dioxide. Such amines are well known in the art and generally contain substituents which sterically hinder the absorption of carbon dioxide. A particularly preferred absorbent is methyldiethanolamine (MDEA). Other suitable selective absorbents of hydrogen sulphide are disclosed in U.S. Pat. No. 4,919,912.
The non-absorbed gas typically forms a purge gas from the process and is typically sent to an incinerator so that its last traces of hydrogen sulphide can be converted to sulphur dioxide. The incinerator typically has a stack through which its combustion products can be vented to the atmosphere.
Although water vapour can be extracted from the reduced gas stream in the absorber vessel itself it is generally preferred to remove water vapour therefrom separately in a discrete vessel intermediate the reduction step and the recycle hydrogen sulphide absorption step.
Preferably, another part of the water vapour depleted gas stream may be returned as an additional recycle stream to the furnace, by-passing the absorption of the hydrogen sulphide. The additional recycle enhances the flexibility of the method according to the invention in handling sour gas streams of different hydrogen sulphide concentrations or of varying compositions.
If desired, between the said steps d) and e) the sulphur depleted gas stream may be subjected to a step of catalytic reaction between sulphur dioxide and hydrogen sulphide therein. If this additional step is performed the apparatus according to the invention additionally includes intermediate the sulphur extraction means and the reactor a bed of catalyst selected to catalyse reaction between sulphur dioxide and hydrogen sulphide in the sulphur-depleted gas stream. Such a catalytic reaction step helps to protect the reduction stage from any surge in the concentration of sulphur dioxide.
Preferably the catalytic reaction between hydrogen sulphide and sulphur dioxide is performed at temperatures above the dew point of sulphur, for example in the range of 160xc2x0 C. to 400xc2x0 C., and particularly 160xc2x0 C. to 300xc2x0 C.
Preferably all the sulphur formed in the catalytic reaction between hydrogen sulphide and sulphur dioxide is allowed to pass into the reduction reactor rather than being extracted from the sulphur-depleted gas stream.
Preferably the mole ratio of hydrogen sulphide to sulphur dioxide in the sulphur-depleted gas stream at the end of step d) is normally at least 4 to 1 and may be as high as 8.5 to 1 or higher. At such high ratios, the sulphur dioxide concentration in the gas stream leaving the sulphur vapour extraction stage can be kept in the order of 1% during normal operation. Accordingly, only a relatively small amount of reduction is required having regard to the hydrogen sulphide content of the feed gas.
The reduction step of the method according to the present invention is preferably performed catalytically at temperatures in the range of 250xc2x0 C. to 400xc2x0 C. The reductant is preferably hydrogen. Typically, the sulphur-depleted gas mixture contains sufficient hydrogen (by virtue of thermal cracking of hydrogen sulphide in the furnace) to reduce all the reducible sulphur species present including sulphur vapour as well as the sulphur dioxide. If needed, however, hydrogen can be supplied from an auxiliary hydrogen generator.
Preferably the same vessel houses the catalyst of the reaction between hydrogen sulphide and sulphur dioxide and the catalyst of the said reduction reaction.
If desired, sour water stripper gas may be premixed with the feed gas upstream of the furnace or supplied separately to the furnace. All the sour water stripper gas is desirably fed to the hottest region of the furnace so as to ensure complete destruction of ammonia.
The sulphur vapour is preferably extracted from the effluent gas stream by condensation.
The water vapour is preferably extracted from the reduced gas stream by direct contact condensation.
If desired, further formation of sulphur vapour may take place intermediate steps (d) and (e) of the method according to the invention.